Dependency injection in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the main benefit of dependency injection in Swift?

Choose the best explanation for why developers use dependency injection in Swift apps.

A It allows objects to create their own dependencies internally for better encapsulation.

B It automatically manages memory for all dependencies without manual code.

C It helps to reduce tight coupling by providing dependencies from outside the object.

D It forces all dependencies to be global singletons for easy access.

💡 Hint

Think about how dependency injection affects how objects get what they need to work.

Dependency injection means giving an object its dependencies from outside, which reduces tight coupling and makes testing easier.

❓ component_behavior

intermediate

What will be printed when this Swift code runs?

Consider this Swift code using dependency injection. What is the output?

iOS Swift

protocol Logger {
    func log(_ message: String)
}

class ConsoleLogger: Logger {
    func log(_ message: String) {
        print("Console: \(message)")
    }
}

class FileLogger: Logger {
    func log(_ message: String) {
        print("File: \(message)")
    }
}

class Service {
    let logger: Logger
    init(logger: Logger) {
        self.logger = logger
    }
    func doWork() {
        logger.log("Work done")
    }
}

let service = Service(logger: FileLogger())
service.doWork()

A No output

B File: Work done

C Work done

D Console: Work done

💡 Hint

Look at which logger instance is passed to the Service.

The Service is initialized with a FileLogger instance, so it prints "File: Work done".

📝 Syntax

advanced

Which option correctly injects a dependency using initializer injection in Swift?

Choose the correct Swift code snippet that injects a dependency via the initializer.

A

class ViewController {
    var service: Service
    init(service: Service) {
        self.service = service
    }
}

B

class ViewController {
    var service: Service
    init() {
        service = Service()
    }
}

C

class ViewController {
    var service = Service()
    init(service: Service) {
        self.service = service
    }
}

D

class ViewController {
    var service: Service
    func inject(service: Service) {
        self.service = service
    }
}

💡 Hint

Initializer injection requires the dependency to be passed and assigned in the init method.

Option A declares the property and assigns it in the initializer, which is the correct pattern for initializer injection.

🔧 Debug

advanced

Why does this Swift code cause a runtime crash?

Examine the code below. What causes the crash when creating ViewController?

iOS Swift

class Service {}

class ViewController {
    var service: Service!
    init() {
        // service is not initialized here
    }
    func useService() {
        print(service!)
    }
}

let vc = ViewController()
vc.useService()

A The useService method is called before the ViewController instance is created.

B The init method is missing a call to super.init(), causing a crash.

C The service property is a constant and cannot be assigned later.

D The service property is declared as implicitly unwrapped optional but never initialized before use.

💡 Hint

Check how the service property is declared and when it is assigned.

The implicitly unwrapped optional service is nil when useService is called, causing a runtime crash.

❓ state_output

expert

What is the final value of counter after running this Swift code with dependency injection?

Analyze the code below. What value does counter have after incrementer.increment() is called?

iOS Swift

protocol Incrementable {
    func increment()
}

class Counter: Incrementable {
    var count = 0
    func increment() {
        count += 1
    }
}

class Incrementer {
    let counter: Counter
    init(counter: Counter) {
        self.counter = counter
    }
    func increment() {
        counter.increment()
    }
}

let counter = Counter()
let incrementer = Incrementer(counter: counter)
incrementer.increment()
let finalCount = counter.count

A 1

B 0

C 2

D Compilation error

💡 Hint

Trace how the count property changes when increment() is called.

The increment() method increases count by 1, so the final value is 1.

Practice

1. What is the main purpose of dependency injection in Swift programming?

easy

A. To provide objects with their dependencies from outside rather than creating them inside

B. To make the app run faster by caching data

C. To automatically generate user interfaces

D. To encrypt data before saving

Solution

1. Step 1: Understand dependency injection concept

   Dependency injection means giving an object the things it needs (dependencies) from outside instead of creating them inside the object.

2. Step 2: Identify the main purpose

   This approach helps keep code flexible, easier to test, and cleaner by separating concerns.

3. Final Answer:

   To provide objects with their dependencies from outside rather than creating them inside -> Option A

4. Quick Check:

   Dependency injection = provide dependencies from outside [OK]

Hint: Dependency injection means giving dependencies from outside [OK]

Common Mistakes:

• Thinking it speeds up app performance
• Confusing with UI generation
• Mixing with data encryption

2. Which of the following is the correct syntax for constructor injection in Swift?

easy

A. class Client { func inject(service: Service) { self.service = service } }

B. class Service { init() { } } class Client { let service: Service; init(service: Service) { self.service = service } }

C. class Client { var service = Service() }

D. class Service { func start() {} }

Solution

1. Step 1: Identify constructor injection pattern

   Constructor injection means passing dependencies through the initializer (init) of a class.

2. Step 2: Check which code passes dependency via init

   class Service { init() { } } class Client { let service: Service; init(service: Service) { self.service = service } } shows a Client class receiving a Service instance through its init method and storing it.

3. Final Answer:

   class Service { init() { } } class Client { let service: Service; init(service: Service) { self.service = service } } -> Option B

4. Quick Check:

   Constructor injection uses init to pass dependencies [OK]

Hint: Constructor injection passes dependencies via init method [OK]

Common Mistakes:

• Creating dependencies inside the class instead of passing them
• Using methods instead of init for injection
• Not storing the injected dependency

3. Given the code below, what will be printed?

protocol Logger { func log(_ message: String) }
class ConsoleLogger: Logger { func log(_ message: String) { print("Console: \(message)") } }
class Client {
  let logger: Logger
  init(logger: Logger) { self.logger = logger }
  func doWork() { logger.log("Work done") }
}
let client = Client(logger: ConsoleLogger())
client.doWork()

medium

A. No output

B. Work done

C. Console: Work done

D. Logger: Work done

Solution

1. Step 1: Understand the Logger protocol and ConsoleLogger class

   ConsoleLogger implements Logger and prints messages prefixed with "Console:".

2. Step 2: Trace the Client usage

   Client is initialized with ConsoleLogger and calls logger.log("Work done"), which prints "Console: Work done".

3. Final Answer:

   Console: Work done -> Option C

4. Quick Check:

   Injected ConsoleLogger prints with "Console:" prefix [OK]

Hint: Injected logger prints message with prefix [OK]

Common Mistakes:

• Ignoring the prefix in print statement
• Assuming no output because of protocol
• Confusing method names

4. Identify the error in the following Swift code using dependency injection:

protocol DataService { func fetchData() -> String }
class RealDataService: DataService { func fetchData() -> String { "Real Data" } }
class ViewModel {
  var dataService: DataService
  init() { }
  func load() -> String { dataService.fetchData() }
}

medium

A. dataService is not initialized before use

B. fetchData() should return Int, not String

C. ViewModel should not have an init method

D. RealDataService does not conform to DataService

Solution

1. Step 1: Check property initialization

   ViewModel has a non-optional property dataService but its init() does not initialize it.

2. Step 2: Understand Swift initialization rules

   Non-optional properties must be initialized before use; missing initialization causes a compile error.

3. Final Answer:

   dataService is not initialized before use -> Option A

4. Quick Check:

   Non-optional properties must be initialized [OK]

Hint: Non-optional properties must be set in init [OK]

Common Mistakes:

• Assuming default nil for non-optional
• Confusing return types
• Ignoring protocol conformance

5. You want to make a Swift class NetworkManager testable by injecting a protocol NetworkService. Which approach best applies dependency injection and allows easy swapping of real and mock services?

hard

A. Create NetworkManager with a fixed real network service inside the class

B. Make NetworkManager inherit from NetworkService and override methods

C. Use global variables to hold the network service instance

D. Define a NetworkService protocol, make NetworkManager accept it via init, and pass either real or mock implementations when creating NetworkManager

Solution

1. Step 1: Understand testability with dependency injection

   To test NetworkManager easily, inject a protocol so you can swap real and mock implementations.

2. Step 2: Identify best practice

   Passing NetworkService via init allows NetworkManager to use any implementation, making testing flexible.

3. Final Answer:

   Define a NetworkService protocol, make NetworkManager accept it via init, and pass either real or mock implementations when creating NetworkManager -> Option D

4. Quick Check:

   Inject protocol via init for flexible testing [OK]

Hint: Inject protocol via init to swap real/mock easily [OK]

Common Mistakes:

• Hardcoding real service inside class
• Using global variables causing tight coupling
• Misusing inheritance instead of composition